There are three main reasons why the soul gets bound to the body and gets in the whirlpool of reincarnation.
Ignorance (Avidya)
It is the primal ignorance that pervades all of the creation. It is the birthplace of all other sorrows/troubles. Spiritual ignorance is fertile soil for all hindrances to soul awareness, whether dormant or active. Thus lack of spiritual knowledge is the birthplace of sorrows. Spiritual ignorance manifests as mistaking the impermanent for the permanent, the impure for the pure, sorrow for joy, and mistaking the human body as the self. Maya/Illusion (and its 3 powers: Filth, Turbulence, and Veil) is responsible for Ignorance.
e.g. [In the darkness, a rope may seem like a snake. We get worried for our life. But when there was full light, we knew it was only rope. It was our ignorance, due to which we suffered with fear. ]
e.g. [Once, in a village, there was an illiterate man and his wife. The man planned to visit Mela (Fair), but he was scared that he might be lost in the fair. He shared his feeling with his wife. His wife suggested that she will tie a rope around his neck. He should check his rope; if the rope is not there, he should know that he is lost; otherwise, if the rope is still in his neck, he is not lost. He agreed and wore a rope around his neck at the fair. He spent all day at the fair, and at night someone asked him why he was wearing a rope around his neck; he told them why. When the man went to sleep, someone removed the rope from his neck and tied it to another man's neck. When the illiterate man woke up in the morning, he checked the rope in his neck but could not find it. Instead, he found it in another person's neck. He started saying that "I am you, I am you." Everyone was laughing at this illiterate man. In the same way, we are Soul, but we think we are bodies due to ignorance. ]
Subtle Desire (Vasana)
Vasana is a karmic impression (past life's desire stored in the subconscious mind) that influences the present behavior of a person. Desires produce impressions on the mind, and impressions generate further desires. The rotation of this never-ending wheel of desires and impressions causes births and deaths.
Desire is the impulse towards an object. It is the root cause of all problems. There is a difference between need, want, wish, and desire. Our days are laden with wants and nights with desires. Thus we remain disturbed all the time but blame others, without understanding that the problem of our pains is our desires. A wish always contains an element of doubt; we are not sure that we may have what we wish for. A want includes an urge; a feeling of compulsion is behind them. Want is the offspring of craving that has its roots in the subtle impressions we carry with us from the past.
There are two kinds of Subtle-Desires - pure and impure. Pure liberate one from rebirth. Impure causes the mind to fluctuate, producing agitation in the mind and affinity for objects. The impure desires are categorized into the following three categories:
Attachment (Raga)
Attachment is the mother of all miseries. It arises when one has achieved something of his desire, and he becomes attached to it by identifying himself with it. Possessing anything is very dangerous because it creates attachment. There is nothing that is ours. We become attached to things and people and think they are ours, but they never belong to us. Ignorance-driven attachment is the cause of birth, death, and rebirth.
Attachment occurs when the self focuses on pleasure and is involved with emotion. In life, people are emotionally attached to many things. People began to pursue human relationships, knowledge, wealth, status, power, or anything capable of enlarging and protecting the fragile individualized existence. However, change is the nature of creation; all objects within it are temporary and thus subject to lose at any moment.
If you want to enjoy life and be happy, learn to practice and understand the philosophy of non-attachment. There is a difference between attachment and love. Attachment is selfish; love is selfless. Attachment brings bondage; love gives freedom. When Spiritual people ask about getting rid of attachment, they are not asking to stop loving or caring; they ask for unselfish love. The greatest happiness in life comes from giving. All the things of the world belong to Providence; use them but do not try to own them.
e.g. [If someone near and dear to us dies, we cry and become sad. But when someone not special to us die, we do not cry. Similarly, if our home is infected with pests, we try to get rid of them by killing them; we do not cry after killing pests. Why? It is because we are attached to our near and dear ones. It also proves that sadness/pain is not in the death; it is present in the attachment.]
e.g. [When we travel on a luxury train, we met certain people and with some of them we develop good relationships and we also enjoy the luxury seats. But when our station came, we have to step off the train; at that point, we cannot develop an attachment for the people met on the train or with the luxury seat. We cannot take the seat with us. If we develop attachment, then we will invite pain.]